1. Field of the Invention
The present invention relates to a control device for a machine tool including a motor for driving a spindle of the machine tool.
2. Description of the Related Art
When cutting using a spindle of a machine tool, a motor that drives the spindle is normally operated under load lower than or equal to its continuous rated load of the motor. Continuous rated load indicates the maximum level of load at which the motor would not overheat even when the load is continuously applied to the motor for infinite time.
However, in the case of heavy-duty cutting, or when machining time is desired to be shortened by increasing, for a short period of time, the load to be applied to the motor, the motor is sometimes rotated under load exceeding the continuous rated load.
In general, for cutting under load exceeding the continuous rated load, a graph showing the allowable load characteristics is provided for each motor to indicate “under n % load, how many minutes the cutting is possible before the motor overheats”. An operator performs a cutting operation by setting up a guide on the basis of the information from the graphs. In an allowable-load characteristics graph, possible cutting time is defined in terms of load. This allows the operator to see how long certain load can be continuously applied to the motor before the motor overheats, by referring to a corresponding allowable-load characteristics graph.
FIG. 9 is a graph showing an example of the allowable load characteristics of a motor. In FIG. 9, the horizontal axis shows a rotation speed of the motor, and the vertical axis shows the load applied to the motor. The 100 percent load indicates the continuous rated load, and, under this load, the motor does not overheat even if the load is continuously applied to the motor for infinite time. The example shown in FIG. 9 indicates that, when the motor is rotated at a low speed, cutting for 2.5 minutes in 10-minute cycle is possible under 200 percent load, and cutting for 1 minute in 10-minute cycle is possible under 230 percent load.
If the motor is rotated under load exceeding the continuous rated load, the motor may generate heat above the allowable heat generation rate, which may eventually cause the motor to break down.
To address this problem, for example, as disclosed in Japanese Patent Application Publication No. 2000-271836, the following control device has been proposed. The control device estimates the heat generation rate of an entire program from estimated values of heat generation rates of a motor at the time of rapid traverse and cutting traverse, and then changes time constants so that the heat generation rate of the entire program would be lower than or equal to the allowable heat generation rate.
Moreover, for example, as disclosed in WO 2005/093942, the following method of preventing overload of a motor has been proposed. In this method, loss (copper loss and iron loss) occurring at a motor is calculated, and, by assuming that loss obtained by subtracting the calculated loss from a rated loss is manageable by the motor, d-axis current corresponding to the obtained loss is passed through the motor.
Allowable load characteristics generally vary among motors. Even by taking into account this respect, if an operator needs to perform a cutting operation while constantly referring to the allowable-load characteristics graphs, such an operation is inefficient and is not likely to be carried out usually. Hence, machining is generally performed by trusting operator's experience and intuition. For example, to prevent a motor from overheating, the motor is sometimes driven with an adequate margin of performance in comparison with the intrinsic performance of the motor. However, such a way of driving is not efficient. Moreover, for example, when machining was performed by trusting operator's intuition, the motor ended up being overloaded and consequently overheating, which stopped the machining, in some cases.
For example, in the invention described in Japanese Patent Application Publication No. 2000-271836 mentioned above, although estimation of the heat generation rate of the motor is made, it is difficult to know how long the machining can be continued before the motor overheats.
For example, in the invention described in WO 2005/093942 mentioned above, although loss including iron loss is estimated, it is difficult to know how long the machining can be continued before the motor overheats.
If it is possible, in machining that involves application of load exceeding the continuous rated load, to know how long the machining can be continued under the current overload to the motor, the motor can be used optimally in terms of the performance of the motor without causing the motor to overheat.